Cellular Physiology — MCQs

Cellular Physiology Indian Medical PG Practice Questions and MCQs

Question 121: What is known as the powerhouse of the cell?

A. Nucleus
B. Cell membrane
C. Mitochondria (Correct Answer)
D. Lysosomes

Explanation: **Explanation:** **Mitochondria** are known as the "powerhouse of the cell" because they are the primary site for **Adenosine Triphosphate (ATP)** production through oxidative phosphorylation. They contain the enzymes for the Krebs cycle (TCA cycle) and the Electron Transport Chain (ETC), converting nutrients into chemical energy that fuels cellular processes. **Analysis of Options:** * **Nucleus:** Known as the "Control Center" of the cell. It houses the genetic material (DNA) and coordinates activities like growth, intermediary metabolism, and reproduction (cell division). * **Cell Membrane:** Acts as a semi-permeable barrier that regulates the transport of substances in and out of the cell and facilitates cell signaling. * **Lysosomes:** Known as the "Suicide Bags" or "Digestive System" of the cell. They contain hydrolytic enzymes that break down macromolecules, damaged organelles, and foreign particles. **NEET-PG High-Yield Pearls:** * **Mitochondrial DNA:** Mitochondria possess their own circular DNA (mtDNA), which is inherited exclusively from the **mother** (Maternal Inheritance). * **Endosymbiotic Theory:** Mitochondria are believed to have originated from aerobic bacteria that entered into a symbiotic relationship with primitive eukaryotic cells. * **Clinical Correlation:** Defects in mitochondrial function lead to **Mitochondrial Myopathies** (e.g., MELAS, MERRF), which typically affect high-energy demanding tissues like the brain and muscles. * **Apoptosis:** Mitochondria play a crucial role in the intrinsic pathway of apoptosis by releasing **Cytochrome c** into the cytoplasm.

Question 122: Which one of the following is a function of lysosomes?

A. Autophagy
B. Autolysis
C. Digestion
D. All of the above (Correct Answer)

Explanation: **Explanation:** Lysosomes are membrane-bound organelles often referred to as the **"suicidal bags"** or the **"digestive system"** of the cell. They contain over 50 different acid hydrolases (e.g., cathepsins, nucleases) that function optimally at an acidic pH (~5.0). 1. **Digestion (Heterophagy):** Lysosomes fuse with endosomes or phagosomes to break down extracellular material (bacteria, food particles) brought into the cell via endocytosis. 2. **Autophagy:** This is the process of "self-eating" where lysosomes degrade worn-out or damaged intracellular organelles (e.g., old mitochondria). This is a vital survival mechanism during nutrient deprivation and for cellular homeostasis. 3. **Autolysis:** In specific physiological or pathological states, lysosomal membranes rupture, releasing enzymes into the cytosol that digest the entire cell. This is a key component of programmed cell death (apoptosis) and post-mortem tissue breakdown. Since lysosomes perform all three functions, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Marker Enzyme:** Acid phosphatase is the classic marker for lysosomes. * **Primary vs. Secondary:** A primary lysosome is "inactive"; it becomes a secondary lysosome (phagolysosome) once it fuses with a vesicle containing substrate. * **Residual Bodies:** Undigested materials that remain within the lysosome (e.g., **Lipofuscin**, the "wear-and-tear" pigment). * **I-Cell Disease:** A deficiency in the Golgi enzyme (phosphotransferase) that tags proteins with **Mannose-6-Phosphate**, leading to lysosomal enzymes being secreted extracellularly rather than being sent to the lysosome. * **Lysosomal Storage Disorders (LSDs):** Result from the deficiency of specific hydrolases, leading to substrate accumulation (e.g., Gaucher’s, Tay-Sachs, Niemann-Pick).

Question 123: Peroxisomes contain what class of enzymes?

A. Oxidative (Correct Answer)
B. Superoxide anion
C. Hydroxyl radical
D. Hypochlorous acid

Explanation: **Explanation:** Peroxisomes (also known as microbodies) are membrane-bound organelles found in almost all eukaryotic cells. They are primarily characterized by the presence of **oxidative enzymes**, such as **catalase, urate oxidase, and D-amino acid oxidase**. The correct answer is **A (Oxidative)** because these enzymes function by removing hydrogen atoms from specific organic substrates in an oxidative reaction that produces hydrogen peroxide ($H_2O_2$). Catalase, the most abundant peroxisomal enzyme, then utilizes this $H_2O_2$ to oxidize other substrates (like phenols and alcohols) or decomposes excess $H_2O_2$ into water and oxygen, protecting the cell from oxidative damage. **Analysis of Incorrect Options:** * **B, C, and D:** These are all examples of **Reactive Oxygen Species (ROS)** or free radicals. While peroxisomes are involved in the metabolism of these substances (specifically neutralizing them), they are the *products* or *byproducts* of metabolic pathways, not the class of enzymes contained within the organelle. Hypochlorous acid (D) is specifically associated with the "respiratory burst" in neutrophils via the enzyme myeloperoxidase. **High-Yield Facts for NEET-PG:** * **Major Functions:** $\beta$-oxidation of Very Long Chain Fatty Acids (VLCFA), bile acid synthesis, and plasmalogen synthesis (essential for myelin). * **Zellweger Syndrome:** A high-yield clinical correlation where a genetic defect in protein import into peroxisomes leads to "empty" peroxisomes. It presents with neurological deficits, hepatomegaly, and early death. * **Adrenoleukodystrophy (X-linked):** Caused by a defect in transporting VLCFAs into peroxisomes, leading to their accumulation and damage to the adrenal glands and white matter of the brain.

Question 124: Action potential is generated in excitable cells. All the following are excitable cells, except?

A. Nerve cells
B. Neuroglial cells (Correct Answer)
C. Glands
D. Muscle

Explanation: **Explanation:** The core concept of this question lies in the definition of **excitability**. An excitable cell is one that can generate and propagate an **action potential** (a rapid reversal of membrane potential) in response to a stimulus. **Why Neuroglial cells are the correct answer:** Neuroglial cells (astrocytes, oligodendrocytes, microglia, etc.) are considered **non-excitable cells**. While they possess a resting membrane potential and can communicate via chemical signaling or gap junctions, they lack a sufficient density of **voltage-gated sodium channels**. Consequently, they cannot generate a self-propagating action potential. Their role is primarily supportive, providing structural, metabolic, and immune defense for neurons. **Analysis of incorrect options:** * **Nerve cells (Neurons):** These are the classic examples of excitable cells. They generate action potentials at the axon hillock to transmit signals over long distances. * **Muscle cells:** All three types (skeletal, cardiac, and smooth) are excitable. In muscle, the action potential triggers the release of calcium, leading to contraction (excitation-contraction coupling). * **Glands:** Many endocrine and exocrine cells (e.g., pancreatic beta cells, anterior pituitary cells) are electrically excitable. They use action potentials to trigger the influx of calcium, which facilitates the exocytosis of hormones or secretory products. **High-Yield Facts for NEET-PG:** * **Resting Membrane Potential (RMP):** In neurons, it is typically -70 mV; in skeletal muscle, it is -90 mV. * **The "All-or-None" Law:** Action potentials follow this principle, whereas graded potentials (like those often seen in glia) do not. * **Glial Function:** While non-excitable, astrocytes play a crucial role in **K+ spatial buffering**, maintaining the excitability of surrounding neurons by absorbing excess extracellular potassium.

Question 125: What is the equilibrium potential for chloride ions?

A. +60 mV
B. +90 mV
C. -90 mV
D. -70 mV (Correct Answer)

Explanation: **Explanation:** The equilibrium potential of an ion is the membrane potential at which the electrical gradient exactly balances the chemical concentration gradient, resulting in no net movement of the ion across the membrane. This is calculated using the **Nernst Equation**. **1. Why -70 mV is correct:** In a typical resting neuron, the extracellular concentration of Chloride ($Cl^-$) is high (~110 mEq/L) while the intracellular concentration is low (~4-10 mEq/L). Because Chloride is a negatively charged anion, its chemical gradient drives it into the cell, while the negative resting membrane potential (RMP) repels it. For most neurons, the equilibrium potential ($E_{Cl}$) is approximately **-70 mV**, which is very close to or identical to the Resting Membrane Potential. This allows $Cl^-$ to stabilize the membrane potential. **2. Analysis of Incorrect Options:** * **A. +60 mV:** This is the equilibrium potential for **Sodium ($Na^+$)**. $Na^+$ is high extracellularly and its influx creates a positive internal environment. * **B. +90 mV:** This value does not correspond to a standard physiological ion equilibrium potential in human neurons. * **C. -90 mV:** This is the equilibrium potential for **Potassium ($K^+$)**. $K^+$ is the primary determinant of the RMP because the resting membrane is most permeable to it. **3. NEET-PG High-Yield Pearls:** * **RMP Determinants:** The RMP is closest to the equilibrium potential of the ion with the highest membrane permeability (Potassium). * **GABA Mechanism:** Inhibitory neurotransmitters like GABA act by opening $Cl^-$ channels. If the cell is depolarized, $Cl^-$ enters the cell to bring the potential back toward -70 mV (hyperpolarization), mediating CNS inhibition. * **Gibbs-Donnan Effect:** The presence of non-diffusible intracellular proteins (anions) influences the distribution of $Cl^-$ across the membrane.

Question 126: Cholera toxin effects are mediated by stimulation of which of the following second messengers?

A. cAMP (Correct Answer)
B. cGMP
C. Ca++ - Calmodulin
D. IP3 / DAG

Explanation: **Explanation:** The correct answer is **A. cAMP**. **Mechanism of Action:** Cholera toxin, produced by *Vibrio cholerae*, consists of an A subunit and a B subunit. The A subunit enters the intestinal epithelial cell and catalyzes the **ADP-ribosylation** of the **Gsα protein**. This modification inhibits the intrinsic GTPase activity of the Gs protein, locking it in a permanently "active" state. This leads to the continuous stimulation of **Adenylyl Cyclase**, resulting in a massive intracellular increase in **cyclic AMP (cAMP)**. High cAMP levels activate Protein Kinase A (PKA), which phosphorylates the **CFTR (Cystic Fibrosis Transmembrane Conductance Regulator)** chloride channels. This causes a profuse secretion of Cl⁻, Na⁺, and water into the intestinal lumen, leading to "rice-water" diarrhea. **Why other options are incorrect:** * **B. cGMP:** This is the second messenger for **Heat-Stable (ST) Enterotoxin** of *E. coli* (via Guanylyl Cyclase activation) and Atrial Natriuretic Peptide (ANP). * **C. Ca++ - Calmodulin:** This system is typically involved in smooth muscle contraction and certain hormone actions (like Oxytocin), but not the primary pathway for Cholera toxin. * **D. IP3 / DAG:** This pathway is utilized by Gq-coupled receptors (e.g., H1, V1, M1, M3). While some toxins affect this, Cholera specifically targets the Gs-cAMP pathway. **High-Yield Clinical Pearls for NEET-PG:** * **Cholera Toxin:** ADP-ribosylation of **Gs** (Stimulatory) $\rightarrow$ $\uparrow$ cAMP. * **Pertussis Toxin:** ADP-ribosylation of **Gi** (Inhibitory) $\rightarrow$ $\uparrow$ cAMP (by preventing inhibition of Adenylyl Cyclase). * **Key mnemonic:** **C**holera = **C**AMP. * The primary cause of death in Cholera is hypovolemic shock due to massive fluid loss.

Question 127: Which of the following is NOT a cell adhesion molecule?

A. Integrin
B. Selectin
C. Cadherin
D. Spectrin (Correct Answer)

Explanation: **Explanation:** Cell adhesion molecules (CAMs) are transmembrane proteins located on the cell surface that facilitate cell-to-cell or cell-to-matrix interactions. They are essential for tissue integrity, leukocyte migration, and signal transduction. **Why Spectrin is the correct answer:** **Spectrin** is not a cell adhesion molecule; rather, it is a major **cytoskeletal protein**. It forms a hexagonal meshwork on the inner surface of the plasma membrane (especially in erythrocytes). Its primary role is to maintain cell shape, provide structural support, and allow for membrane flexibility. Mutations in spectrin lead to clinical conditions like **Hereditary Spherocytosis**. **Analysis of other options:** * **Integrins (Option A):** These are heterodimeric receptors that primarily mediate **cell-matrix** adhesion (e.g., binding to fibronectin or laminin). They also play a role in "inside-out" signaling. * **Selectins (Option B):** These are carbohydrate-binding proteins (L, E, and P-selectins) that mediate the initial **"rolling"** phase of leukocyte adhesion to vascular endothelium. * **Cadherins (Option C):** These are **calcium-dependent** homophilic adhesion molecules (e.g., E-cadherin) that are crucial for maintaining stable cell-to-cell junctions like desmosomes and adherens junctions. **High-Yield NEET-PG Pearls:** * **Calcium Dependency:** Cadherins and Selectins are calcium-dependent, whereas the Immunoglobulin (Ig) superfamily and Integrins are generally calcium-independent. * **Leukocyte Extravasation:** Remember the sequence: **Rolling** (Selectins) → **Activation** (Chemokines) → **Adhesion/Tethering** (Integrins/ICAM-1) → **Diapedesis** (PECAM-1). * **Clinical Link:** Loss of E-cadherin is a hallmark of **Epithelial-Mesenchymal Transition (EMT)** in cancer metastasis.

Question 128: Which of the following is involved in transport across the nucleus?

A. Importins
B. Local signals
C. Ran proteins
D. All of the above (Correct Answer)

Explanation: **Explanation:** Nucleocytoplasmic transport is a highly regulated process occurring through the **Nuclear Pore Complexes (NPCs)**. It involves a coordinated interaction between transport receptors, targeting signals, and molecular switches. 1. **Importins (Option A):** These are specialized transport receptors (karyopherins). **Importins** bind to cargo proteins in the cytoplasm to shuttle them into the nucleus, while **Exportins** facilitate movement out of the nucleus. 2. **Local Signals (Option B):** Proteins destined for the nucleus contain specific amino acid sequences called **Nuclear Localization Signals (NLS)**. Conversely, those exiting the nucleus possess **Nuclear Export Signals (NES)**. These signals are essential for recognition by transport receptors. 3. **Ran Proteins (Option C):** Ran is a small **GTPase** that provides the directionality and energy for transport. It exists in two states: **Ran-GTP** (high concentration in the nucleus) and **Ran-GDP** (high concentration in the cytosol). This gradient ensures that cargo is loaded and unloaded in the correct compartment. Since all three components—receptors (Importins), signals (NLS/NES), and the molecular switch (Ran)—are indispensable for nuclear transport, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Energy Source:** Nuclear transport is an active process requiring energy derived from **GTP hydrolysis**, not direct ATP hydrolysis. * **Size Limit:** Small molecules (<40-60 kDa) can pass via passive diffusion, but larger proteins require the active transport mechanism described above. * **Clinical Correlation:** Defects in nuclear transport receptors are linked to neurodegenerative diseases (e.g., ALS) and certain cancers, where transcription factors are inappropriately sequestered in the cytoplasm.

Question 129: Fixed time is required for all of the following steps of the cell cycle, except?

A. S phase
B. M phase
C. G1 phase (Correct Answer)
D. G2 phase

Explanation: ### Explanation The cell cycle duration varies significantly between different cell types, and this variation is almost entirely due to the **G1 phase**. **1. Why G1 phase is the correct answer:** The G1 phase (Gap 1) is the most variable part of the cell cycle. Its duration depends on external factors like nutrient availability, cell size, and growth factors. Cells can remain in G1 for a few hours, days, or even years (entering the G0 or quiescent phase). Because it acts as the primary "decision-making" period where the cell determines whether to commit to division at the **Restriction Point (R point)**, its timing is not fixed. **2. Why the other options are incorrect:** Once a cell passes the G1 restriction point and enters the **S phase**, the subsequent steps follow a relatively "fixed" or constant time course to ensure genomic integrity: * **S phase (DNA Synthesis):** Usually takes 8–10 hours in most human cells to ensure precise replication of the entire genome. * **G2 phase (Gap 2):** Usually lasts 2–5 hours, providing a fixed window for checking DNA errors and preparing proteins for mitosis. * **M phase (Mitosis):** The shortest and most constant phase, typically lasting only 0.5–1 hour. **3. NEET-PG High-Yield Pearls:** * **G0 Phase:** A modified G1 phase where non-dividing cells (like neurons or skeletal muscle) reside. * **Generation Time:** The total time from one mitosis to the next. In rapidly dividing human cells (like intestinal epithelium), it is about 24 hours. * **Regulation:** The transition from G1 to S is regulated by **Cyclin D-CDK4/6** complexes. * **Labile Cells:** Cells that never enter G0 and have a very short G1 (e.g., bone marrow, GI tract). * **Permanent Cells:** Cells that stay in G0 indefinitely (e.g., Neurons, Cardiac myocytes).

Question 130: Which of the following depicts the cell cycle sequence accurately?

A. G0 - M - G2 - S - G1
B. G0 - G1 - G2 - S - M
C. G0 - G1 - S - G2 - M (Correct Answer)
D. G0 - G1 - S - M - G2

Explanation: **Explanation:** The cell cycle is a highly regulated, ordered sequence of events that results in cell division. The correct sequence is **G0 → G1 → S → G2 → M**. 1. **G0 (Quiescence):** A resting phase where the cell has exited the cycle and is not actively dividing (e.g., neurons). 2. **G1 (Gap 1):** The cell grows, synthesizes RNA, and prepares proteins for DNA replication. 3. **S (Synthesis):** The critical phase where **DNA replication** occurs, doubling the DNA content. 4. **G2 (Gap 2):** A period of further growth and "proofreading" of replicated DNA before division. 5. **M (Mitosis):** The phase of nuclear and cytoplasmic division. **Why other options are incorrect:** * **Options A & B:** These suggest that the M phase occurs before G1 or that G2 occurs before S. DNA must be replicated (S phase) *before* the cell prepares for division (G2) and actually divides (M). * **Option D:** This places the M phase before G2. The G2 phase is essential as a pre-mitotic checkpoint to ensure DNA integrity; division cannot occur immediately after synthesis without this quality control. **High-Yield NEET-PG Pearls:** * **Duration:** The **S phase** is typically the most constant in duration, while **G1** is the most variable. * **Checkpoints:** The **G1 to S transition** (regulated by Cyclin D/CDK4 and Rb protein) is the "Restriction Point"—the most critical point for cell cycle commitment. * **Labelling:** Radioactive thymidine is used to label cells specifically in the **S phase**. * **Permanent Cells:** Neurons, skeletal, and cardiac muscle cells remain in **G0** and do not regenerate.

Practice by Chapter

Cell Membrane Structure and Function

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Membrane Transport Proteins

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Cellular Energetics and Metabolism

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Mitochondrial Function

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Cell Volume Regulation

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Cellular Responses to Stress

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Calcium Signaling

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Cell Cycle and Regulation

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Cellular Aging

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Apoptosis and Cell Death

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